Electrophoretic imaging system

ABSTRACT

An electrophoretic image duplicating process wherein a suspension of particles in an insulating carrier liquid are placed in an electroded system. One electrode has on its surface an imagewise pattern of a material which will on application of electrical field alone inject charge into the particles causing the particles to migrate imagewise. In operation of the process an electrical field is established between the electrode carrying the imagewise pattern and a second electrode with the result that an imagewise pattern of particles corresponding to the imagewise pattern carried by the electrode is formed on the second electrode. Apparatus for carrying out the process is also described.

United States Patent Wells et a1.

Nov. 4, 1975 1 1 ELECTROPHORETIC IMAGING SYSTEM [75] Inventors: John B. Wells, Brighton; Robert W.

Gundlach, Victor, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: June 27, 1973 [211 App]. No.: 374,216

[52] US. Cl. 427/14; 427/25; 118/637 [51] Int. Cl. ..(;03(i13/10;G03G 13/14; (11136 15/10; G036 15/14 [58] Field of Search 117/37 LE; 96/1 R, 1 LY,

96/1 P, 1 E; 355/10, 3 P; 118/637; 427/14, 25

[56] References Cited UNITED STATES PATENTS 3,383,993 5/1968 Yeh 117/37 LE 3,384,566 5/1968 Clark l H 117/37 LE 3,474,019 10/1969 Krieger et 111m... ,1 96/1 LY 3,689,399 9/1972 Ota 96/1 PE 3,729,334 4/1973 Snclling 4 l l l 117/37 LE 3,741,761 6/1973 Cantarano r 117/37 LE 3,769,009 10/1973 Wells et a1. 117/37 LE Primary Examiner-Michael Sofocleous Attorney, Agent, or Firm .lames .l. Ralabate; David C. Petre; Richard A. Tomlin [57] ABSTRACT An electrophoretic image duplicating process wherein a suspension of particles in an insulating carrier liquid are placed in an electroded svstem. One electrode has on its surface an imagewise pattern of a material which will on application of electrical field alone in* ject charge into the particles causing the particles to migrate imagewise.

In operation of the process an electrical field is established between the electrode carrying the imagewise pattern and a second electrode with the result that an imagewise pattern of particles corresponding to the imagewise pattern carried by the electrode is formed on the second electrode. Apparatus for carrying out the process is also described.

15 Claims, 3 Drawing Figures US. Patent Nov. 4, 1975 /8 ll lo i I60 f Z", W i /4 10 a i no I5 ELECTROPHORETIC IMAGING SYSTEM BACKGROUND OF THE INVENTION This invention relates to image duplicating systems. More specifically, this invention concerns an electrophoretic image duplicating system.

The use of electrical fields or charge patterns to form images is well-known. For example, in xerography such as described in US. Pat. No. 2,297,691 to C. F. Carlson, photoconductors are used on which an electrostatic charge pattern is formed. This charge pattern may be developed by contacting the charge pattern with a developer liquid containing particles of colorant which are drawn to the charge pattern by electrostatic attraction. Many variations of the above process exist. In one variation the surface of a photoconductor is ex posed to a light image while an electrical field is applied across the photoconductor. The photoconductor does not accept a charge in illuminated conductive areas but does in dark insulating areas thus forming an electrostatic charge pattern. Particles of colorant in, e.g., a liquid developer are drawn to or precipitated on the surface of the photoconductor in the charged areas forming a visible image.

The main disadvantage of these prior art systems using photoconductors is that, conventionally, only one image is obtained from each exposure. Further, process speed is controlled by a number of factors such as time of charging and discharging with visible light and photosensitivity.

There are prior art processes for producing multiple images such as printing processes, e.g., lithography. These processes, however, usually require complex and expensive preparation and are seldom considered for short run processing.

SUMMARY OF THE INVENTION It is an object of this invention to provide a method of image duplicating which overcomes the above-noted disadvantages.

It is another object of this invention to provide a novel electrophoretic image duplicating system.

It is another object of this invention to provide an electrophoretic image duplicating system which can provide a relatively large number of copies from one imagewise exposure, or no imagewise exposures.

It is another object of this invention to provide an image duplicating system utilizing a relatively inexpensive and rapidly formed master.

The above objects and others are accomplished in accordance with this invention by providing an electrophoretic imaging system wherein a charge carrier generating and injecting material is placed in imagewise configuration on the surface of a conducting substrate material. This member will hereafter be referred to as an injecting master. A suspension of, e.g., colored par ticles in an insulating liquid is coated on the injecting master. An electrical field is then imposed between the conductive substrate of the injecting master and a second electrode held in contact with the free surface of the suspension. Application of field causes the charge injecting material to inject charge into or onto the particles causing the particles to migrate to the oppositely Charged second electrode, that is, the electrode in contact with the free surface of the suspension. The particulate image thus formed may be fixed in place or transferred as desired.

The critical component of the present invention is the charge injecting material. The preferred charge in jecting material is a material capable of high generation and injection of charge carriers in response to an electrical field. To facilitate handling of the injecting mate rials, they may be dispersed in a binder. Preferred binders are those materials capable of accepting and transporting charge carriers generated by the injection material and themselves being capable of allowing injection into other materials.

No imagewise exposure is used in or required by the present process except where desired to form the original injecting master. the phrase injection is intended to describe the present process mechanism in which application of do electrical field across a material causes that material to inject charge carriers into a medium which is within interaction range of the material. Many of the materials which are capable of generating and injecting charge carriers are photoconductive when used in xerographic processes. It should be noted, however, that the present process does not rely on the use of light or other radiation to which the photoconductive materials respond.

Any suitable injecting material may be used in the present invention and these include organic as well as inorganic materials. Typical organic materials include pigments such as quinacridones, carboxamidcs, car boxanalides, triazines, benzopyrrocolines, anthraquinones, azos, salts and lakes of compounds derived from 9 phenyl-xanthenc, dioxazines, lakes of fluorescein dyes, substituted pyrene, bisazos, phthalocyanines and mixtures thereof. Specific organic pigment materials are listed at columns 8 and 9 ofU.S. Pat. No. 3,384,488 issued May 2], 1968, the disclosure of which is incorporated herein by reference.

Further organic and inorganic materials include di ethyl thiourea, allyl thiourea and those materials listed at Column 9, line 67 through Column 11, line 1 ofU.S. Pat. No. 3,384,488, the disclosure of which is incorporated herein by reference.

Other inorganic materials include cadmium sulfide, cadmium sulfoselenide, zinc oxide, zinc sulfide, sulphur, selenium, mercuric sulfide, lead oxide, lead sulfide, cadmium selenide, titanium dioxide, indium trioxide and mixtures thereof. Mixtures of organic and inorganic materials may also be used.

The above materials may be used alone or combined with a binder. Any suitable film-forming material may be used as a binder and includes materials such as ther moplastic or thermosetting resins. Preferred binder materials are those capable of accepting and transporting charge carriers over a relatively long distance.

Typical materials include polymeric materials such as polyvinylcarbazole, poly-l-vinylpyrene, polymethyiene pyrene and N-substituted polymeric acrylic acid amides of pyrene and mixtures thereof. Typical nonpolymeric materials include carbazole; N-ethylcarbazole; N-phenylcarbazole; pyrene; tetraphene; l-acetylpyrene; 2,3-benzochrysenc; 6,7 benzopyrene; bromopyrene; l-ethylpyrene; l-methylpyrene; perylene; Z-phenylindole; tetracene; picene; l,3,6,8-tetraphenylpyrene; chrysene; fluorene; fluorenone; phenanthrene', triphenylene', l,2,5,6-dibenzanthracene; l,2,3,4-dibenzanthracene; 2,3-benzopyrene; 2,3 ben zochrysene; anthraquinonc; dibenzothiophene', naphthalene; mixtures thereof and mixtures of non-polymeric and polymeric materials.

The particles which may be dispersed in the liquid may be insulating, semi-conductive or conductive and may be made up of two or more components. Since it is essential that the particles be capable of accepting and retaining charge injected from the injecting master, it has been found that the surface of the particles should be made of a material which has a bulk resistivity of at least I ohm-cm and preferably ohm-cm or greater. There is no known upper limit of resistivity in that particulate dyed plastics having resistivities of greater than 10 ohm-cm having been found to be operablev It is desirable to use particles of relatively small size because small particles provide more stable suspen sions and provide images of higher resolution and covering power than would be possible with larger particles. It is thus preferred that the particles be less than I or 2 microns in average cross section although particles up to about 5 microns and larger may readily be used. No lower limit of particle size is known.

The concentration of particles dispersed in the liquid depends on the density of the final image desired, the use to which the image is to be put and the size of the particles, the solubility of added dispersants, and other factors generally known to those skilled in the art of ink or plastic coating formulation. For example, when finely-divided dyed resinous materials are dispersed in mineral oil or kerosene. from about 1 part by weight to about 50 parts by weight ofa resinous particulate mate rial dispersed in lOO parts by weight liquid provide sat isfactory images.

The carrier liquid may comprise any suitable insulating liquid or solid which may be converted to a liquid at the time of imaging. Typical insulating materials include decane, dodecane, tetradecane, kerosene, molten paraffin, molten beeswax or other molten thermoplastic material, mineral oil, silicone oils such as dimethyl polysiloxane, fluorinated hydrocarbons and mixtures thereof.

The substrate for the injecting master and the second electrode may comprise any suitable conductive material. Typical transparent conductive materials include cellophane, conductively coated glass such as aluminum or tin oxide coated glass, or metallized transparent plastic materials such as polyester film. Other typical conductive materials include metals and conductive rubber.

The second electrode and other electrodes where used preferably have an insulating web or layer over their outer surface to help support the relatively high fields used in the process and also to reduce or prevent particle oscillation.

Typical insulating materials include paper, plastic coated paper, cellulose acetate, nitro cellulose, polysty rene, polytetrafluoroethylene, and related fluorinated polyolefins, polyvinyl fluoride, polyurethane and poly ethylene terephthalate.

In accordance with this invention, images may be made of virtually any material. For the production of colored images, the particles may be dyed thermoplas tic materials which are especially suitable for full color transparency or opaque image formation. One advantage of using dyed thermoplastic materials is that brightly colored materials may be used which are readily fused to form a fixed final image. To produce a polychrome image, two or more images may be made and transferred to a common receiver in register and fused thereon. For other applications the particles may 4 be chosen to be reflective glass beads. luminescent phosphors, resin coated ferromagnetic materials, pig ments, reflective resin coated metal particles, microcapsules containing liquids or other materials, catalytic particles or particles otherwise specially formulated for specific end uses when in the form of shaped patterns.

The injecting master may be formed by any convenient method such as by painting the injecting material in image configuration onto a conductive substrate. The material can be sprayed through a stencil, brushed through a stencil or applied by any other suitable mechanical means. For these processes it may be desirable to dissolve or suspend the injecting material in a volatile material with or without a binder.

Further, since many of the injecting materials respond to electrical field and light, processes such as photoelectrophoresis as shown in U.S. Pat. No. 3,3 84,566 to H. E. Clark or manifold imaging as shown in U.S. Pat. No. 3,707,368 to W. G. Van Dorn may be used to form injecting masters. Conventional xerography may also be used to form an injection master by developing the electrostatic charge pattern using injecting material and transferring the injecting material to a conductive substrate. It is necessary only to provide an image of the injecting material on a conductive substrate and to provide a reasonable bond between the material and substrate so that the material does not become dislodged in use. To aid in bonding or for other reasons such as surface protection, it may be desirable to apply a thin layer ofa nonconductive material to the conductive substrate before formation of the injecting master image thereon. Where such layer is used, it must be relatively thin or be of a material which will not interfere with field application sufficiently to render the injecting material inoperative.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. IA and 1B are schematic representations of the image-forming steps of the present invention.

FIG. 2 is a diagrammatic side view of a simple exemplary imaging apparatus for producing multiple copies in a continuous imaging mode in accordance with the present invention.

Referring now to FIG. IA, conductive electrode 1 has bonded to its surface in image configuration injecting material 2 forming an injecting master. Injection material 2 may be particles dispersed in a binder, a single phase material or other material as explained above. Particles 4 dispersed in insulating liquid 5 are deposited relatively uniformly over the surface of the injecting master by, for example, application of corona discharge prior to bringing electrode 6 into contact with the free surface of the liquid suspension. It is preferred to use this pre-deposition step since it provides images of higher quality and lower background than when no pre-deposition step is used.

Referring now to FIG. 113, when switch 7 is closed, electrical potential from source 8 is applied across the injection master and suspension causing particles 4 which were within interaction range of the injecting material 2 to move away and adhere to electrode 6. It should be pointed out here that the sizes and shapes of the various components and distances have been distorted for purposes of explanation. For example, the layer of particles may be several particles deep, further the electrodes may be forced into virtual contact which means that large particles may be in actual contact with both the charge injection master and the opposing electrode 6 at the same time. The image formed on electrode 6 may be fixed in place or transferred to an image receiving surface such as paper. Transfer may be aided, e.g., by application of electrical field or by adhesive pick-off.

Referring now to FIG. 2, there is seen conductive drum 11 having on its surface injection master image I2 shown in greater detail in FIG. 1. Imaging suspension 13 made up of particles 14 dispersed in insulating liquid 15 is coated on surface 12 by roller 16 which is immersed in suspension 13.

Roller 17 made up of conductive core 18 covered by insulating layer 19 is connected to source of potential which is used to apply an electrical field across suspension 13, driving the particles 14 in suspension 13 to the surface of injecting master 12 leaving a relatively clear layer of liquid on the outer surface. This step reduces background on the image formed on image receiver sheet 22 which is, for example, paper. Roller 17 may be replaced by a rod, conductive doctor blade, corona source or other potential applying device.

Receiver sheet 22 entrained over conductive drum 24 is used to receive the image. Conductive drum 24 is connected to one side of potential source 10, the opposite terminal being connected to ground and drum 1]. In apparatus as shown, potentials ranging from 300 to 7000 volts do or more may be used depending on the particular cell structure, i.e., whether insulating layers are present in the gap.

In the dark injection operation when suspension 13 reaches the area of the nip formed by surfaces 12 and 22, the electrical field causes injection material image areas not shown on surface 12 to inject charge into the particles 4, causing them to migrate through the liquid layer and deposit on surface 22 in imagewise formation. The image may be fused in place or transferred as desired.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically illustrate the improved electrophoretic image duplicating process of this invention. The examples are intended to illustrate various preferred embodiments of this invention.

Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I Approximately 1 part of metal-free phthalocyanine is suspended in a solution of about 5 parts of polyvinylcarbazole in 50 parts by weight toluene. The suspension is ball milled until the phthalocyanine particles are uniformly suspended and have an average particle size of about l2 microns. This suspension is coated onto the conductive surface of a conductive metal plate in image configuration so that when dry, the image is approximately IO microns thick. This image is used as the injection master in Examples IIV.

Approximately 50 parts by weight of finely divided resincoated carbon black made by dispersing about 5 parts by weight carbon black in 95 parts by weight ofa polyester resin having an average molecular weight of about 6500 is dispersed in about 50 parts by weight Sohio Odorless Solvent 3454, a mixture of kerosene fractions. This suspension is coated on the injection master, that is, the conductive surface and the phthalocyanine-polyvinyl carbazole image. From this point on the image thus formed is kept in the dark or under safelight conditions unless indicated otherwise because the phthalocyanine is responsive to light. A source of high potential is connected to a roller electrode which has a I inch diameter steel core and on its surface a inch layer of polyurethane having a resistivity of 1 X 10 ohm-cm forming a total diameter of about 2.5 inches. A paper sheet is placed over the polyure thane surface to receive the images. The other lead of the source of high potential is connected to the conductive image bearing plate and to ground.

With a potential of 5000 volts applied, the roller being negative with respect to the grounded injection master, the roller is caused to traverse the resinated carbon black suspension at a rate of about 2 inches/- second. On completion of roller traverse, an image made up of the resinated carbon black particles is found adhering to the paper covered roller. The resinated carbon black suspension on the injection master is then replaced and the imaging step repeated forming a second image. The reinking and imaging steps are repeated 20 times with no apparent drop off in image quality.

EXAMPLE I] The experiment of Example I is repeated using in place of the carbon black suspension a suspension of about l2 parts by weight magenta dyed melamine formaldehyde resin particles in about 40 parts by weight Sohio Odorless Solvent 3454. Magenta images are formed as in Example 1.

EXAMPLE III The experiment of Example I is repeated using a white mineral oil in place ofthe Sohio with results simi lar to those obtained in Example 1.

EXAMPLE IV The experiment of Example I is repeated using in place of the carbon black suspension a suspension of about 12 parts by weight of 5 micron particles of iron oxide overcoated with melamine formaldehyde resin in about 50 parts by weight of Sohio 3454. Images are produced as in Example I. Images made herein may be used as magnetically readable images.

EXAMPLE V A mixture of 1 part of metal-free phthalocyanine and 10 parts of melamine formaldehyde resin are milled in about 50 parts by weight Sohio 3454 until particle size is reduced to about l-2 microns. The mixture is coated imagewise onto a conductive metal flat substrate to a thickness dry of about 10 microns and heated to remove the solvent and to adhere the phthalocyanine resin to the substrate. The image is used as in Example I with comparable results.

EXAMPLES Vl-VIII The experiments of Examples lI-lV are repeated using the injection master of Example V with comparable results.

EXAMPLE IX An injection master is prepared by coating a conductive plate imagewise with a mixture of I part of polyvinylcarbazole and I0 parts by weight toluene to a thickness of about l0 microns. The solvent is removed providing a polyvinylcarbazole image on the substrate. The

7 injection master thus formed is used as in Example I. By reinking between each image formation cycle as in Example l, 30 images are made without degradation of image quality.

EXAMPLES X-XIl The experiment of Example IX is repeated using the suspensions of Examples ll-lV with results comparable to Example IX.

EXAMPLES Xlll-XXIV All of the above experiments of Examples l-Xll are repeated in the presence of ambient illumination of normal room light levels with similar results.

Although the above examples show the injecting master image being placed directly on the conductive substrate, an insulating web may be placed between the injecting material and the conductive substrate. For example, the process has been shown to be operable with the injecting material being on the surface of 3 mil Mylar, a polyester available from duPont and the Mylar in turn being backed by a conductive material. This embodiment requires the removal of accumulated charge from the insulating web for rapid recycling which may be accomplished, for example, by the use of a corona discharge.

Although specific components and proportions trini trofluoreneone been described in the above examples, other materials as listed above, where suitable, may be used with similar results. In addition, other materials may be added to the various layers to synergize, enhance or otherwise modify their properties. For example, a waxlike material may be added to the suspension which, on evaporation of the liquid, will aid fixing. Further, charge control agents such as tricresylphosphate or pentachlorophenol may be added to the suspension. Materials such as triphenylamine and trinitrofluoroenone may be added to the injecting material or suspension to enhance charge transport.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

l. A method of forming images which comprises the steps of:

a. providing in imagewise configuration a layer of an injecting material on a conductive substrate to form an injecting master, said injecting material being capable of generating and injecting charge carriers in response to electrical field alone;

b. providing a second electrode member;

c. providing a layer of a suspension of particles in an insulating liquid, wherein said particles have a surface having a bulk resistivity of at least about 10 ohm-cm; and

d. applying an electrical potential difference between said conductive substrate and said second electrode of magnitude sufficient to cause imagewise migration of particles as a result of charge carrier injection from said injecting material until an image is formed.

2. The method of claim 1 wherein said injecting material comprises a charge carrier generating and injecting material dispersed in a binder.

3. The method of claim I wherein said injecting material comprises a charge carrier generating and injecting material in solid solution in a binder.

4. The method of claim 1 wherein said injecting material is homogeneous.

5. The method of claim I wherein said injecting material is inorganic.

6. The method of claim 1 wherein said injecting ma terial is organic.

7. the method of claim I wherein said injecting material is provided on the surface of an insulating web.

8. The method of claim 1 and further including prior to step (d),

providing a third electrode member and applying an electrical potential difference between said third electrode and said conductive substrate of suffi cient magnitude to cause at least a portion of said particles in said suspension to deposit on said injecting master.

9. The method of claim 8 wherein said third electrode is a source of corona.

10. The method of claim 1 wherein said particles have a diameter of up to about 5 microns.

11. The method of claim 1 wherein said particles have a diameter of about 2 microns or less.

12. Apparatus for forming images which comprises:

a. a first conductive member having an imagewise layer of an injecting material on a surface, said in jecting material being a material capable of generating and injecting charge carriers in response to electrical field alone, said surface being adapted to support a layer of a suspension of particles in an insulating carrier liquid;

b. a second conductive member;

c. means for moving said second conductive member into contact with suspension on said first conductive member, and

d. means for applying an electrical potential difference between said first conductive member and said second conductive member of sufficient magnitude to cause particle migration in said suspension in response to charge carrier injection from said imagewise layer of injecting material.

13. The apparatus of claim 12 wherein said second conductive member has an insulating surface for contacting suspension on said first conductive member.

14. The apparatus of claim 12 and further including a third conductive member positioned to apply an electrical field across suspension supported on said first conductive member prior to contacting suspension supported on said first conductive member with said second conductive member and an electrical potential source for applying an electrical potential difference between said third conductive member and said first conductive member, said potential being of a magnitude sufficient to cause particles in said suspension to deposit on said first conductive member.

15. The apparatus of claim 14 wherein said third conductive member is a source of corona. 

1. A METHOD OF FORMING IMAGES WHICH COMPRISES THE STEPS OF: A. PROVIDING IN IMAGEWISE CONFIGURATION A LAYER OF AN INJECTING MATERIAL ON A CONDUCTIVE SUBSTRATE TO FORM AN INJECTING MASTER, SAID INJECTING MATERIAL BEING CAPABLE OF GENERATING AND INJECTING CHARGE CARRIERS IN RESPONSE TO ELECTRICAL FIELD ALONE, B. PROVIDING A SECOND ELECTRODE MEMBER, C. PROVIDING A LAYER OF A SUSPENSION OF PARTICLES IN AN INSULATING LIQUID, WHEREIN SAID PARTICLES HAVING A SURFACE HAVING A BULK RESISTIVITY OF AT LEAST ABOUT 10**5 OHM-CM, AND D. APPLYING AN ELECTRICAL POTENTIAL DIFFERENCE BETWEEN SAID CONDUCTIVE SUBSTRATE AND SAID SECOND ELECTRODE OF MAGNITUDE SUFFICIENT TO CAUSE IMAGEWISE MIGRATION OF PARTICLES AS A RESULT OF CHARGE CARRIER INJECTION FROM SAID INJECTING MATERIAL UNTIL AN IMAGE IS FORMED.
 2. The method of claim 1 wherein said injecting material comprises a charge carrier generating and injecting material dispersed in a binder.
 3. The method of claim 1 wherein said injecting material comprises a charge carrier generating and injecting material in solid solution in a binder.
 4. The method of claim 1 wherein said injecting material is homogeneous.
 5. The method of claim 1 wherein said injecting material is inorganic.
 6. The method of claim 1 wherein said injecting material is organic.
 7. the method of claim 1 wherein said injecting material is provided on the surface of an insulating web.
 8. The method of claim 1 and further including prior to step (d), providing a third electrode member and applying an electrical potential difference between said third electrode and said conductive substrate of sufficient magnitude to cause at least a portion of said particles in said suspension to deposit on said injecting master.
 9. The method of claim 8 wherein said third electrode is a source of corona.
 10. The method of claim 1 wherein said particles have a diameter of up to about 5 microns.
 11. The method of claim 1 wherein said particles have a diameter of about 2 microns or less.
 12. APPARATUS FOR FORMING IMAGES WHICH COMPRISES: A. A FIRST CONDUCTIVE MEMBER HAVING AN IMAGEWISE LAYER OF AN INJECTING MATERIAL ON A SURFACE, SAID INJECTING MATERIAL BEING A MATERIAL CAPABLE OF GENERATING AND INJECTING CHARGE CARRIERS IN RESPONSE TO ELECTRICAL FIELD ALONE, SAID SURFACE BEING ADAPTED TO SUPPORT A LAYER OF A SUSPENSION OF PARTICLES IN AN INSULATING CARRIER LIQUID, B. A SECOND CONDUCTIVE MEMBER, C. MEANS FOR MOING SAID SECOND CONDUCTIVE MEMBER INTO CONTACT WITH SUSPENSION ON SAID FIRST CONDUCTIVE MEMBER, AND D. MEANS FOR APPLYING AN ELECTRICAL POTENTIAL DIFFERENCE BETWEEN SAID FIRST CONDUCTIVE MEMBER AND SAID SECOND CONDUCTIVE MEMBER OF SUFFICIENT MAGNITUDE TO CAUSE PARTICLE MIGRATION IN SAID SUSPENSION IN RESPONSE TO CHARGE CARRIER INJECTION FROM SAID IMAGEWISE LAYER OF INJECTING MATERIAL.
 13. The apparatus of claim 12 wherein said second conductive member has an insulating surface for contacting suspension on said first conductive member.
 14. The apparatus of claim 12 and further including a third conductive member positioned to apply an electrical field across suspension supported on said first conductive member prior to contacting suspension supported on said first conductive member with said second conductive member and an electrical potential source for applying an electrical potential difference between said third conductive member and said first conductive member, said potential being of a magnitude sufficient to cause particles in said suspension to deposit on said first conductive member.
 15. The apparatus of claim 14 wherein said third conductive member is a source of corona. 